docs(comments): rewrite comments across all crates to the guidelines

Sweep every first-party crate source (1956 .rs files) to the project comment
guidelines: delete redundant restatements, decorative banners, change
narration, and end-of-line comments; keep and tighten the crucial ones
(invariants, bug rationale, SAFETY blocks, ported-source attribution).

No functional code changed. Every edit is proven comment-only against the
prior tree by a comment-stripping lexer (string/char/raw-string aware) plus a
separate doctest-fence check. Where removing a comment made rustfmt or clippy
want to re-lay-out adjacent code, the minimal triggering comment is restored so
code tokens stay byte-identical.

Gates green: cargo fmt --all --check (0 diffs), cargo check and cargo clippy
--workspace --all-targets (0 warnings).

Adds scripts/check_codegen_comment_guidelines.py — the enforcement gate for
these guidelines (flags banners, end-of-line comments, change narration, and
commented-out code).

crates/codegen/kigi-crash-handler/tests/integration.rs

@@ -33,7 +33,7 @@ fn run_scenario(scenario: &str, crash_dir: &Path) -> (std::process::ExitStatus,
     )
 }
 
-// ── Subprocess entry point ──────────────────────────────────────────────
+// Subprocess entry point
 
 /// This test is `#[ignore]`d so it only runs when invoked as a subprocess
 /// by the parent test via `run_scenario`. The `CRASH_TEST_SCENARIO` env
@@ -43,7 +43,8 @@ fn run_scenario(scenario: &str, crash_dir: &Path) -> (std::process::ExitStatus,
 fn subprocess_entry() {
     let scenario = match std::env::var("CRASH_TEST_SCENARIO") {
         Ok(s) => s,
-        Err(_) => return, // not a subprocess invocation
+        // not a subprocess invocation
+        Err(_) => return,
     };
     let crash_dir = std::env::var("CRASH_TEST_DIR").expect("CRASH_TEST_DIR");
     let crash_dir = std::path::PathBuf::from(crash_dir);
@@ -138,7 +139,7 @@ fn subprocess_entry() {
     }
 }
 
-// ── Parent test cases ───────────────────────────────────────────────────
+// Parent test cases
 
 #[test]
 fn handler_does_not_interfere_with_tokio_runtime() {